#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<ll> vll;

const int DY[] = {-1,0,1,0};
const int DX[] = {0,1,0,-1};

int W, H, M[102][102], vis[102][102];
bool ok = false;

void dfs(int y, int x, int py, int px){
    if(vis[y][x] == 1)ok = true;
    if(vis[y][x])return;
    vis[y][x] = 1;
    rep(i, 4){
        int ny = DY[i] + y, nx = DX[i] + x;
        if((ny != py || nx != px) && M[y][x] == M[ny][nx]){
            dfs(ny, nx, y, x);
        }
    }
    vis[y][x] = 2;
}

int main(){
    cin >> W >> H;
    rep(i, H)rep(j, W)scanf("%d", &M[i + 1][j + 1]);
    rep(i, H)rep(j, W)dfs(i + 1, j + 1, -1, -1);
    puts(ok ? "possible" : "impossible");
}